This invention relates to a method of producing nonwoven fabrics not substantially comprising openings through a treatment with high velocity water streams. More particularly, the invention relates to an improvement in a method of producing nonwoven fabrics wherein a fibrous web (or a batt: this interchangeability of term applies to all of the same term, fiber web, appearing in the following statements) is treated on a water impermeable supporting member with water jet streams ejected from a nozzle.
In a conventional method of producing nonwoven fabrics of the type wherein configuration of nonwoven fabrics is maintained by an individual fiber entanglement carried out through a treatment with water jet streams, two methods are available with reference to selection of a member for supporting the fibrous web in the water jet streams treatment: one being a method wherein a water permeable supporting member consisting of a porous screen or of a porous plate is employed; the other being a method wherein a water impermeable supporting member consisting of a roll or of a plate is employed.
Under the method of employing the water permeable supporting member, since the water jet streams ejected onto the fibrous web pass through the supporting member, draining treatment of the water streams can be easily made and the fibrous web may be treated with good stability.
However, the water streams still are of a considerable pressure even after passing through the fibrous web and the supporting member, no effective utilization of energy of the water streams in the entangling treatment is provided. Such tendency increases particularly in proportion to the decrease in the weight of the fibrous web per square meter and affects the efficiency. Accordingly, under the method of employing the water permeable supporting member, neither improvement in the production speed nor reduction in the production costs can be attained. Furthermore, since this method necessitates the treatment with extremely high pressure of water jet streams, large scale production facilities are involved and is therefore not economical.
Under the method of employing the water impermeable supporting member, water jet streams ejected onto the fibrous web pass through the fibrous web, collide with the surface of the supporting member and are converted into rebound streams to act again, provided that the question of the draining treatment is effectively resolved, wherefore the fiber entanglement can effectively be carried out by an interaction between the jet streams and the rebound streams. Accordingly, this method does not accompany the disadvantages as seen in the method of employing the water permeable supporting member discussed above. It is to be noted, however, that since the water streams do not permeate through the supporting member under the method of employing the water impermeable supporting member a question as to how the draining treatment should be made remains; in the case where the draining is not carried out sufficiently, fibers float in the water and said fibers are acted upon by the jet streams whereby the energy of the jet streams is sharply diminished, entangling treatment of the fibers is hindered, the fibrous web is caused to be disarranged and the stability in the treatment thereof is lost. For these reasons, the method discussed does not make it possible to provide nonwoven fabrics with superior property. In this connection, employment of a roll, a curved plate, a grooved plate or the like as a water impermeable supporting member have heretofore been proposed but a mere selection of such member does not ably overcome the problems. None of the conventional literature regarding the method of employing the water impermeable supporting member discloses concrete measures to effectively solve the various problems discussed above. In fact, the method of employing the water impermeable supporting member has not yet been successfully put into industrial practice, so far as the inventors of the present invention know.
Accordingly, the basic object of the present invention resides in the provision of a method of producing whereby various problems discussed above can effectively be overcome and a nonwoven fibrous sheet with superior property can industrially be produced in large quantity. Other objects will be understood from the following statements.
In order to attain the above object, the present invention provides a method of producing nonwoven fabrics wherein: in a method of producing nonwoven fabrics wherein a fibrous web is guided onto a water impermeable supporting member and said fibrous web is subjected to water jet streams ejected from the nozzles which are arranged with intervals in a manner to face the surface of said fibrous web and to run across the width thereof whereby entangling treatment of individual fibers of said fibrous web is carried out; the improvement which comprises employing a fibrous web weighing from 15 to 100 g/m.sup.2 as said fibrous web, guiding said fibrous web onto a first supporting member consisting of a smooth-surfaced water impermeable endless belt, carrying out a preliminary entangling treatment with the water jet streams ejected from said nozzles arranged with respect to said first supporting member, guiding said fibrous web entangled to a certain degree through said preliminary entangling treatment onto each of second supporting members consisting of a plurality of smooth-surfaced water impermeable rolls disposed with intervals, and carrying out said entangling treatment with the water jet streams ejected from said nozzles each arranged with respect to each of said second supporting members.
According to the present invention, treatment of the fibrous web is carried out with a ejection of the water jet streams on the second supporting member consisting of a plurality of water impermeable rolls multistagedly and parallely arranged in order to provide effective draining treatment and to obtain nonwoven fabrics with superior property. It is to be noted, however, that originally the fibrous web is formed with a slight entanglement of fibers and the configuration thereof which is maintained only with such mere entanglement may be liable to distortion or breakage even with minor external force. Accordingly, it may happen that when the fibrous web is guided onto the initial water impermeable roll for treatment, it is damaged by the water jet streams drained in front and in the rear of the roll and the treatment thereof becomes impossible. For this reason, preliminary entangling treatment is arranged to be carried out on the first supporting member consisting of a water impermeable belt which is capable of supporting the fibrous web with good stability, whereby necessary strength to transfer toward the said roll is provided.
In the entangling treatment of the fibers with ejection of the water jet streams on said belt and on each of said rolls which are to function as a water impermeable supporting member for the fibrous web, it happens that, where draining treatment is insufficient, the fibers floating in the water are acted upon by the water jet streams whereby energy of the water jet streams is sharply diminished, entangling treatment of the fibers are hindered, the fibrous web is caused to be disarranged and the stability in the treatment thereof is lost, as discussed previously. For this reason, in a preferred embodiment, an arrangement is made so that an average quantity of supply of water in a direction of width to be ejected onto each water impermeable supporting member is less than 400 cc/sec.cm, more preferably less than 30 cc/sec.cm. An average quantity of supply of water in a direction of width, referred to above, indicates the value obtained through F/w where F corresponds to a total flow quantity ejected into a single water impermeable supporting member and W corresponds to the effective width of nozzles on said supporting member. Where such value is more than 40 cc/sec.cm, it is not possible to obviate the disadvantageous situation or result mentioned above.
In a preferred embodiment, jet pressure of the water jet streams is less than 35 kg/cm.sup.2, more preferably 15 to 30 kg/cm.sup.2 and where such pressure is more than 35 kg/cm.sup.2, movement of individual fibers within the fibrous web becomes great and thereby the fibrous web is caused to be in disorder and to be uneven in the fiber entanglement whilst where the pressure is less than 7 kg/cm.sup.2, no effective production of nonwoven fabrics with superior property is possible, howsoever a long time treatment is carried out with respect to the fibrous web or the nozzles are brought close to the fibrous web to the extent that they nearly get in touch with the fibrous web.
The water jet streams are ejected from the nozzles. Types of the nozzles to be employed for this purpose are: for example, a nozzle of the type wherein plural jet holes are formed in a pipe at regular intervals, a nozzle of the type wherein jet holes are formed in a basic plate at regular intervals and the basic plate with such construction is incorporated into, for instance, a distributing pipe, or the like. In a preferred embodiment, the vertical cross section configuration of the jet holes of the nozzle consists of a portion with a gradually reduced diameter toward the jet holes and a portion with a small diameter extending straightforwardly in order that the resistance of water streams relative to the jet holes is lessened and the loss of pressure toward the jet holes is diminished, wherein where the length of the latter portion is supposed to be L and the diameter thereof is supposed to be D, the ratio L/D is set to be less than 4/1 or preferably less than 3/1. In case where the former portion is not of the configuration mentioned above and the ratio L/D is set to be more than 4/1, straightforward transferability of the water streams from the jet holes is equal to the case wherein said ratio L/D is set to be less than 4/1. Since, however, the resistance of water streams increases, the loss of pressure toward the jet holes becomes large. Further, the configuration of the water jet streams represents columnar streams of water the diameter of the jet holes is arranged to be 0.05 to 0.2 mm and the nozzles comprise the jet holes with intervals from 0.5 to 10 mm.
It is necessary that the belt and the individual rolls referred to above as the water impermeable supporting member for fibrous webs should be of hard surfaces sufficient enough to prevent that energy of the water jet streams is adsorbed by the deformation of the supporting members and thereby the efficiency in the fiber entanglement is lowered. In a preferred embodiment, the hardness of surface is set to be more than 50.degree., more preferably over 70.degree., according to the regulation of K6301Hs under JIS(Japanese Industrial Standard). As far as such hardness is maintained, metal, rubber, plastic and the like may be used solely or in combination to provide a multiple construction.
As the fibrous web, any one of the conventional fibers used generally in the past for the production of woven fabrics, nonwoven fabrics or the like may be used. As the web configuration, any type of random, parallel, cross web or the like may be employed. However, since the present invention is directed to a method employing a water impermeable supporting member, a fibrous web of the type with weight of 15 to 100 g/m.sup.2, preferably 20 to 60 g/m.sup.2 is used, such that energy of the water jet streams may efficiently be provided for the fibrous web. Where such weight is less than 15 g/m.sup.2, irregularity of the fibrous web occurs and nonwoven fabrics with substantial uniformity can not be obtained. Where such weight is more than 100 g/m.sup.2, sufficient effect to be enjoyed by the use of the water impermeable supporting member can not be obtained. In a preferred embodiment, a random web formed by a card provided with at least one condensing roll which is arranged between a doffer and a comber in such a manner that the circumferential surface speed is substantially lower than that of the doffer, is used as the fibrous web. With the employment of this random web, it is possible to provide a nonwoven fabric which has no difference in its lengthwise or crosswise tenacity, the fibers of which are oriented in three dimensional direction and which is richer in its bulkiness than the web obtainable from a conventional random card.
According to the method of the present invention, draining treatment can be sufficiently carried out. Since various problems involved in the method of using the water impermeable supporting member as explained above can all be resolved, efficient treatment of the fibrous web with water jet streams can be made and hence desired objects may be attained. The nonwoven fabrics obtained through the method according to the present invention do not substantially comprise openings and the fibers are intricately and firmly entangled in three dimensional direction. Accordingly, the nonwoven fabrics provided by the present invention are superior in tensile strength bulkiness and flexibility. This means that the nonwoven fabrics provided by the present invention are of excellent suitability as a constitutional element of sanitary goods, particularly such as sanitary napkins, disposable diapers or the like which are used in contact with the human body and humors. It is also possible to employ the nonwoven fabrics by the present invention for a wide variety of fields of general goods covering such as industrial filter, a wiper, a pillow case or the like.